docs(#124): CHANGELOG 末尾の比較リンク定義を 0.5.1 用に更新

[unreleased] の比較 base を v0.5.0 → v0.5.1 に置換し、
[0.5.1]: compare/v0.5.0...v0.5.1 を新規追加。他バージョンの
比較リンク定義との一貫性を回復する。

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: etoyama

CHANGELOG.md

@@ -127,7 +127,8 @@ Version bumps and maintenance releases. See git history for details:
 - YAML direct edit resilience (extra field preservation + corrupt file isolation)
 - SQLite FTS5 full-text search index
 
-[unreleased]: https://github.com/etoyama/insight-blueprint/compare/v0.5.0...HEAD
+[unreleased]: https://github.com/etoyama/insight-blueprint/compare/v0.5.1...HEAD
+[0.5.1]: https://github.com/etoyama/insight-blueprint/compare/v0.5.0...v0.5.1
 [0.5.0]: https://github.com/etoyama/insight-blueprint/compare/v0.4.4...v0.5.0
 [0.4.1]: https://github.com/etoyama/insight-blueprint/compare/v0.4.0...v0.4.1
 [0.4.0]: https://github.com/etoyama/insight-blueprint/compare/v0.3.0...v0.4.0